声学仿真软件可以描绘出喷气式发动机主要部件发出的声响，比如发动机舱、涡轮转子等。

在飞机的设计中，提高燃油经济性、降低排放量的要求日趋紧迫，力度之大以至于忽视了飞机设计过程中另一重要因素：噪音。

噪音同燃油经济性和减排问题一样，是限制飞机飞行的重要因素。全球各地的机场对起飞和降落小时数都进行了限制，以此保护机场周围地区不受过多的飞机噪音干扰。而同时，搭乘飞机出行的普及和全球经济对红眼航班的依赖和需求的不断增加，促使航班的数量正不断持续增加。

当全世界的政府都表示他们愿意使用各种手段限制飞行时间，以此来管理机场的噪音，例如实行夜间禁飞、加收费用、限制噪音水平、采取限额制度等措施，而航空产业也正着手设计更安静的机型来应对噪音问题。

在制造商能够生产出更安静的飞机之前，他们必须向工程师提供必要的设计流程与技术工具。以现有发展模式大多数的生产商都没有充分考虑声学方面对此的影响，以至于在生产过程中无法大幅度减少噪音的问题。因此从一种新机型概念诞生伊始，航空工程师就必须要权衡设计创新所带来的影响，权衡噪音、燃油经济性以及其他所有其他因素。

噪音反作用

20世纪70年代，人们就已经开始忧虑飞机噪音过多，因而此后，相关规定也变得越来越严苛。

超音速协和喷气式客机大概就是噪音反作用最有名的受害者了。由于噪音太大，欧洲与北美有关当局允许这款英法联合机型降落的地点就相对较少。服役30年后这款机型最终退出历史舞台，这中间有很多因素影响，不过权威部门的限制也是导致这款机型被停止开发的原因之一。

房屋业主的抱怨致使美国国会于1968年授权美国联邦航空局（FAA）对新机型设计制定噪音标准。FAA按照噪音标准将飞机划为三个等级(阶段)，并安排计划逐步淘汰噪音最大的机型。

欧洲航空安全组织（Eurocontrol）预测到2018年，欧洲空中交通将增加16%。欧盟国家对II阶段的飞机采取了严格的控制措施，并考虑提出议案，淘汰在欧盟各机场服役的噪音最大的III 阶段机型。

根据欧盟的信息显示出，自20世纪70年代起飞机的安静程度已经提升了75%，但即便如此，以上措施的采取也势在必行。

一台喷气式发动机在进气和排气时的声学曲线

目标：安静高效

即使对于飞机噪音的顾虑一直存在，但是航空燃油的成本又不停地增加，所以制造更加高效的飞机就成了当务之急。但燃油经济性措施与降噪之间存在矛盾，因此噪音控制也显的尤为重要。

例如，反向旋转的引擎消耗的燃油比传统引擎更少，但产生的噪音更大。过去，让飞机部件和机身更加安静就意味着要靠增加重量来减少震动。不过现在，航空制造企业正在不断试验使用复合材料和高强度金属来减少重量。不但质量轻了，震动也相应的减少了。

不过，平衡噪音和燃油经济性也不是不可能实现的。工程师克服了更大的困难，例如除了使用更新更轻的材料之外，还透过对机身、发动机舱的设计以及绝缘分布等方式来降低飞机的噪音。

但是这样也是有条件的。工程师必须要在设计流程进入原型机阶段之前，就早早的能够判断出噪音减少的效果应用。如果到了后期再做更改那代价就会变的非常高昂，所以如果工程师的设计即使只完成了50%，他们手里的降噪选择也是十分有限。

同样，如果不加测试就将降噪的想法转换成设计，那么在原型机阶段有可能会发现这个设计妨碍飞机性能，或者发现结果得非所愿。为了在流程初期创新的同时将犯错风险降到最小，工程师就需要工具来模仿单个组件的噪音，比如发动机舱的噪音，这样他们才能看到新设计有什么效果、整个飞机的噪音又是如何。

喷气发动机的分析网格提供了基础声学分析。

声学仿真软件工具应用于航空业已经20年了，但即使是这样，还是有几家公司在早期设计阶段没有很好地将它融合到流程中。

不过空中客车和罗罗都是例外。他们1999年与FFT公司（Free Field Technologies）（现在叫做MSC软件公司）合作开发了Actran声学仿真软件，可以模拟整个系统的声音，而且能同时处理发动机噪音和机身噪音（由机身周围的湍流引起）。

但最早的声学仿真软件太过复杂，不是每一个工程师都能掌握。是空客能过15年以来不断改进，将这个软件进行了“大众化”。如今，这个声学仿真软件已经完全融入了他们的设计流程中。

空客的任何一位工程师都可以在设计流程中运用声学仿真软件测算。他们可以利用Actran软件对诸如速度、温度、高度等参数值进行变更测算，并获得最终结果报告。因此空客工程师在流程初期就可以大致知道哪一个设计最有希望。

设计进行到尾声，他们也可以调整参数来获得最佳性能。这个系统可以消除猜测臆想和不必要的重复，通过不断模拟仿真不同想法和确认设计的正确性，避免设计晚期犯下代价高昂的错误。

不管哪家公司在声音管理方面遇到什么样的挑战，至少空客向他证明了将声学仿真应用贯穿于整个设计流程是可行的。这一方法给工程师提供了他们需要的声学仿真信息，并通过这些有效信息来制造噪音更低的机型。业界需要更安静的飞机来促进成长，但如果成长的代价是导致机场周围居民生活质量下降，那这也实现不了。

Jean-Louis Migeot博士和Jean-Pierre Coyette博士为《航天工程》撰写了此文，他们是一家MSC软件公司——FFT（Free Field Technologies）的联合创始人。

Acoustic simulation software can depict the sound profiles of key jet-engine assemblies such as the nacelles and turbine spinner.

Greater fuel efficiency and low emission requirements have grown into such an urgent imperative in aircraft design that it often overshadows an equally significant factor in air travel—noise.

Noise is as much a limiting factor on air travel as fuel efficiency and reduction of emissions. Airports around the world restrict takeoff and landing hours to protect surrounding areas from excessive jet aircraft noise. This is at the same time that air travel’s popularity and the world economy’s reliance on overnight air freight are increasing and driving demand for more flights, not fewer.

The aerospace industry has responded to the noise challenge by designing quieter aircraft as governments all over the world have demonstrated they are willing to manage airport noise by restricting flight times in many different ways, such as curfews, surcharges, noise level limits, and quotas.

Before aerospace manufacturers can produce quieter aircraft, however, they must provide engineers with the necessary design processes and technology tools. The current development model in effect at most manufacturers does not fully consider acoustics until too late in the process to achieve significant noise reduction. To balance noise and fuel economy with all of the other considerations that go into designing aircraft, aerospace engineers must be able to weigh the effects of their design innovations from a new aircraft’s inception.

The noise backlash

Concerns about excessive aircraft noise surfaced in the 1970s and have spurred increasingly stringent regulations ever since.

The supersonic Concorde jet liner is probably the most famous victim of the noise backlash. The British-French aircraft was restricted to relatively few landing sites by authorities in Europe and North America concerned about excessive noise. Though many factors influenced the aircraft’s demise after almost 30 years in operation, those restrictions contributed to decisions not to develop a new version.

Homeowner complaints led the U.S. Congress to give the U.S. FAA authority in 1968 to set noise standards for new airplane designs. The FAA designated three generations (stages) of aircraft by noise level and laid out a schedule for phasing out the loudest.

In Europe, where the air travel safety agency Eurocontrol predicts a 16% increase in air travel by 2018, European Union nations have strict controls on Stage II aircrafts and are considering a proposal to phase the loudest Stage III aircrafts out of fleets that service EU airports.

All this comes in spite of the fact that aircraft have become 75% quieter since the 1970s, according to the European Union.

The acoustic profile of a jet engine’s fan intake and exhaust is shown.

The goal: quiet efficiency

Even as noise concerns were swirling around air travel, the seemingly endless increases in aviation fuel costs made more efficient aircraft an immediate necessity. This is significant to noise control because fuel-efficiency measures can conflict with efforts to make planes quieter.

Counter-rotating jet engines, for example, consume less fuel than conventional engines but they’re also louder. Making parts and fuselages quieter has traditionally meant adding weight to reduce vibration. Today, however, aerospace companies are experimenting with composites and high-strength metals to reduce weight. Lightness can increase vibrations.

Nevertheless, it isn’t impossible to balance noise and fuel efficiency. Engineers have surmounted higher obstacles. Even with new and lighter materials, there are opportunities to reduce noise through the shape of the fuselage, engine nacelle design, and insulation distribution, for example.

There’s a proviso, however. Engineers must be able to determine how their noise-reduction adaptations will perform long before the design reaches the prototype stage. Late-stage changes are prohibitively expensive, so engineers have limited options for noise control if a design is even 50% finished.

By the same token, incorporating a noise-saving idea into a design without testing it runs the risk of finding out during prototyping that it affected the aircraft’s performance, or didn’t yield the results it was supposed to. To innovate early in the process while minimizing the risk of costly mistakes, engineers need tools that can simulate the noise profile of a single component, such as the nacelle, so they can see the immediate effect of their innovation, and the overall noise profile of the aircraft.

The analysis mesh of a jet engine provides the basis for acoustic analysis.

Acoustic simulation software tools have been in the aircraft industry for 20 years, though at all but a very few companies they have been poorly integrated into early stage design processes.

Airbus and Rolls-Royce are among the exceptions. They teamed up in 1999 with Free Field Technologies (now an MSC Software company) to develop Actran acoustic simulation software that can model entire systems and handle both engine noise and airframe noise (as induced by turbulent flows around the aircraft).

The earliest acoustic simulation software was too complex for every engineer in the process to learn, but Airbus has worked steadily over the past 15 years to “democratize” it. Today, acoustic simulation is fully integrated into its design processes.

Any engineer in Airbus’ design process can initiate an acoustic simulation calculation. They change values for parameters such as speed, temperature, and altitude in a simulation model and submit it for calculation. Actran runs the simulation and reports the results to engineers. Airbus engineers can use Actran at the beginning of the process to get a broad idea of which design is the most promising.

As they get closer to a final design, they can adjust the parameters for optimal performance. This system eliminates guesswork and needless iteration. It avoids costly late-stage errors through constant simulation that reveals when an idea is going awry.

Whatever a company’s sound management challenges, Airbus shows that it’s possible to infuse acoustic simulation into design processes from beginning to end. This approach gives engineers the acoustic simulation intelligence they need to create quieter aircraft. The industry needs quieter aircraft so it can grow, which it won’t be allowed to do if it reduces the quality of life around airports.

Dr. Jean-Louis Migeot and Dr. Jean-Pierre Coyette, co-founders of Free Field Technologies (FFT), an MSC Software company, wrote this article for Aerospace Engineering.